Ever noticed that, in movies, the slightest tap of a bullet on a car or a helicopter will cause it to burst into flames, without the presence of an accelerant or any source of fuel? Finally there's a physical effect that can explain why a metal bullet slamming into a metal car can cause it to turn into a fireball. We'll walk you through the vaporific effect.

In movies anything can explode. Sure, there are limitations. The thing exploding can't be next to the hero unless he has a way to escape, and it also can't be next to the villain, unless it's the final scene. But there is at least the potential for explosive combustion in everything from a building site to a old car that's out of gas to a horse-drawn carriage. Everything is basically a firecracker with its fuse lying tantalizingly close to a lit match. For those of you who have scoffed at watching explosions happen whenever a bullet scrapes against a car door, finally there is a physical effect that will let you comfort yourself.

The thrillingly-named vaporific effect pops up in movies a great deal more than it pops up in real life, but it can and does happen. All it needs is a high-velocity projectile to go crashing into a metal object. Neither object necessarily even has to have anything we might consider as combustible in it.

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The principle is the same one that causes the frequent explosions and fires that plague flour and sugar factories. What often stops a fire is not lack of fuel but lack of ready oxygen. This is one of the reasons why kindling is used to get fires going. The tiny strands of fiber are surrounded by air, and so have access to enough heat to allow them to burn easily. Tiny particulates of carbon, like sugar or flour dust, burn even quicker. If they're evenly distributed in the air, one tiny speck sets fire to the next, and the entire area explodes with a massive amount of heat. When a projectiles strikes a metal object, dust in the area can serve the same function. The strike itself causes pieces of both objects to scatter through the air, and they can catch fire as well.

But why is metal so necessary to this reaction? There's a reason why this happens with metallic objects more than others. The metals we're familiar with look placid and malleable, and when they are lumped together, they are. When they're finely divided, ordinary materials like titanium, aluminum, and iron are easily combustible. Some metals react explosively with oxygen, and others with water vapor. Finely grated metal shavings, or wires, are often required to be shipped in containers filled with inert gases, to cut down on the likelihood that they spontaneously catch fire. When a metal object is bent or shattered, and fine shavings are cast out into the air, there is a good chance that they'll catch fire and cause all the little fragments around them to do the same.

So the next time you see a hero blindly fire a gun at a gangster's car, and see it blow up like it was made of matchsticks, just tell yourself it's the vaporific effect. Or that it's a bad movie.